Process and pneumatic device for detecting the bending angle of a plate sheet in a bending press

ABSTRACT

The invention refers to the field of bending presses for plate sheets and more precisely it refers to a process and a device for detecting the bending angle in real time when the press punch operates on the sheet placed above the matrix die. The process provides for the generation of at least two flows of compressed air in at least two matrix die points that are totally or partially covered by the plate sheet. The pressure variation during the bending step is measured and such variation is compared with sample pressure values. The device provides for a compressed air system with a pressure reducer that supplies four orifices and a servo-controlled device that performs a compensation.

The present invention deals with a process and a pneumatic device fordetecting the bending angle of a plate sheet in a bending press.

As known, bending presses can have different construction variations.The most widespread ones are composed of a moving table supporting apunch and a fixed table supporting a matrix die.

The moving table can translate in a vertical plane along two risers onwhich two hydraulic cylinders are assembled for this movements.

Matrix die and punch have different shapes depending on the bendingangle that has to be obtained, and therefore are both or only one ofthem interchangeable, provided that they are compatible.

The plate sheet is rested onto the matrix die and the punch drops overit.

One of the major problems of bending presses is verifying, at the end ofthe bending step, the bending angle and in particular whether such anglecoincides with the desired angle.

In fact, it is known that the plate dose not keep the angle imprinted bypunch and matrix die due to the elastic return of the plate itself.

The elastic plate return can be determined with difficulty, since itdepends on three major variable factors:

-   -   plate thickness, which is not constant in all plate sheet        points;    -   material of which the plate is composed;    -   lamination direction of the plate sheet.    -    Such factors can change for the same sheets and for the same        workings, so that it is necessary, for every bend plate sheet,        to verify whether the bending angle coincides with the desired        angle and possibly intervene with a new pressing action.

Several systems, of the mechanical and of the optical type, are alreadyknown for detecting the bending angle, and the most accurate onesprovide for carrying out the measure on four points.

The mechanical system disclosed, for example, in U.S. Pat. No.4,131,008, IT-A-1293374, IT-A-1294147, provide for feeler means todetect the bending angle on the plate intrados or extrados, composed ofone or two elastically yielding and independent forks.

In case of two forks, they can be placed one inside the other or side byside.

The fork heads or bits are adapted to be arranged in contact with thesheet plate.

The two elastically yielding forks are connected on their lower side toa related position transducer that communicates with a logical dataprocessing unit that manages the bending press.

Another system of the mechanical type is disclosed in DE-A-10006512 inwhich, for detecting the bending angle, two levers are used with theirfulcrum onto the matrix die and which, through their rotation movement,operate on a signal transducer.

Another system for detecting the bending angle is the one manufacturedby the Belgian Company LVD that provides for a moving arm, placed besidethe matrix die, which, supported by two linkages, and during the bendingstage, arranges sensor means in contact with one of the two wings of thediverging plate.

The sensor means are coaxially moving with respect to such arm andprovide detection data to the bending press logical unit.

These mechanical system have the inconvenience that they cannot beapplied to all types of matrix dies, in particular matrix dies withnarrow slots, since physically there is no room for inserting the forks.

Moreover, the fork movement can be constrained by the presence of dirtor metallic debris that can always be found in environments where metalsare worked.

The optical system, see WO-A-01/28706, provides for at least one sourceof a light beam that is used for projecting two spots or a linearsegment onto part of the sheet to be controlled.

The bending angle is given by the distance between the projected spots,the known beam incidence angles, and the known distance betweendetection planes.

The passing channels of the two light beams create a punch weakening;moreover, such channels can be easily occluded by debris or impuritiesthat can be found on the plates thereby impairing their reading ormaking it unreliable.

It also known from patent EP 1083403 a method and device for detectingthe bending angle of a plate. EP 1083403 shows a bending die havingorifices in the face and slot side. In order to detect the bendingangle, the pressure difference in an orifice is measured. With the helpof tables, formulas, etc the value of the pressure difference istransferred in a value of an angle.

Object of the present invention is ensuring the bending angledetermination with a system that is able to avoid any occlusion orreading obstacle.

These and other objects are all obtained by the process and the deviceof the present invention, that is characterised by what is provided inthe enclosed claims.

These and other characteristics will be better pointed out by thefollowing description of some embodiments shown, merely as anon-limiting example, in the enclosed tables of drawing, in which:

FIG. 1 schematically shows the present device applied to a matrix die ofa bending press before starting the bending step;

FIG. 2 shows the device in FIG. 1 in an intermediate bending step;

FIG. 3 shows the device in FIG. 1 in a final bending step;

FIG. 4 shows, in a block diagram form, a preferred embodiment of themeasuring system with auxiliary devices that increase its accuracy.

With reference to FIG. 1, reference 1 designates a matrix of a bendingpress in a cross section, and reference 10 designates the matrix dieslot.

The punch 2 is provided over the matrix die, while reference 3designates a plate sheet that has to be bent along a desired line andthat must reach a certain bending angle.

Two pairs of orifices, symmetrically arranged with respect to thevertical pressing axis 4, can be noted in this matrix die section.

Each pair comprises a first orifice 5 and a second orifice 6, which aresupplied through respective channels 7 and 8 obtained in the matrix diebody, through a compressed air system.

The compressed air system, whose line 9 is shown, provides for aprecision pressure reducer RP that always keeps the pressure constant ata pre-set value, with a symmetrical and balanced piping system, adaptedto sent the same air flow rate to the four orifices when they arecompletely open.

In the most general case, on each line a pressure detector is inserted,which transmits its own signal to a signal transducer which is able tocommunicate with a data processing unit that is able to process theposition assumed by the plate, and therefore the plate bending angle,every time during its bending and particularly till the punch ceases itsaction and goes away from the matrix die, while the plate elasticallyreturns to its final bending angle.

The orifices location is preferably the shown one and more precisely thefirst orifice 5, called external orifice, is vertically arranged along aperpendicular direction to the plate sheet before its bending and issubstantially on the plane matrix die surface next to the slot.

The second orifice 6, called internal orifice, is arrangedperpendicularly to the slot face and exits onto this face at a certaindistance from the first orifice.

The two orifices therefore are on two surfaces that get in contact withthe plate at different times when the plate is bent with its maximumpossible angle.

As shown in FIG. 1, the plate sheet, at the beginning of the pressingcycle, completely clogs the orifice 5 and the detected pressure will bemaximum, while the orifice 6 will be completely free with a minimumpressure, since air will be able to freely go out to the outsideenvironment. When starting the pressing step, see FIG. 2, the orifice 5will start to be freed while the orifice 6 will start to close due tothe plate bending.

In the final step, FIG. 3, the orifice 5 will be completely free whilethe orifice 6 will be totally or partially closed.

In a preferred embodiment, the pressure detectors are not of theabsolute type, but of the differential type, and detect the pressuredifference that occurs in the duct going to the internal orifices withrespect to the ducts going to the external orifices.

During the elastic return step, the plate bending angle measure musttake into account both angles against which the plate rests on the twomatrix die edges.

In order to do this, two separate differential pressure transducers canbe used, one for each matrix die edge, and their signals can beprocessed in a combined way.

In the preferred embodiment being shown, the same ducts that mutuallyconnect the external orifices and that mutually connect the internalorifices inside the matrix die, pneumatically realise the mean ofmeasures of the two angles, based on symmetry, and therefore a singledifferential pressure sensor is used.

The angle measure can be obtained by measuring the detected pressuredifference, through a calibration table that compensates for theunavoidable system lack of linearity, and takes into account thedependence on the supply air pressure.

In a preferred system implementation, by modifying the pneumatic circuitas shown in FIG. 4, it is possible to avoid main measuring errors, dueto the pressure reduced inaccuracy and the pressure sensor scale errors.

Branching from the two ducts connected to the differential pressuresensor TDP supplied by a pressure regulating unit RP, a compensatingdevice 12 is added, composed of a pair of symmetrical orifices 13 with ashutter 14 connected to a servo-positioning device 15 adapted toprogressively change the air passage section of the two orifices alongopposite directions, in a way that is quite similar to what occurs onthe matrix die edge when bending the plate.

If the servo-positioning device is controlled by the pressure differencedetected by the differential pressure sensor TDP, it moves since it istaken to zero, since the air passage section difference in matrix dieorifices is perfectly compensated by the section difference ofcompensating device orifices.

In this way, the plate angle is bi-univocally related to the positionreached by the shutter, which can be very accurately detected throughvarious systems known per se.

In the described example, the position is detected with an absoluteencoder, not shown in the figures, and connected to the numeric presscontrol.

After an initial calibration, for example performed with calibrateddihedral angles placed on the matrix die, the numeric control builds atable from which it obtains, at any time, the plate bending angle,starting from the shutter position, or vice versa.

It can be noted that the residual error due to differential sensor zerodrift can be detected and compensated by the system upon every bendingcycle, by simply shutting off the common compressed air supply and bytaking into account the residual value provided by the sensor.

If, instead, the shutter is kept in a fixed position, corresponding tothe minimum angle reached at the end of bending, the immediate measureof angle variations can be obtained, due to the plate thicknessdifferences and the elastic return, by measuring pressure variationsprovided by the differential pressure sensor TDP around the zero value.

Such application is feasible both by using a matrix die as tool in amachine or alternatively a punch; in FIG. 4, a punch tool UP, which canreplace the matrix die tool UM, is alternatively connected with dashedlines as device for detecting the bending angle.

The description made an explicit reference to controlling pressures inorifices, but, according to a possible variation, the parameter to beverified could be the compressed air outflow rate, such rate valuechanging depending on orifice clogging by the plate sheet.

A single pressure measuring system can be used with different pairs oforifices obtained in different matrix dies or punches, by every timeselecting solenoid valves on respective pneumatic supply ducts.

1-11. (canceled)
 12. Process for detecting the bending angle of a platesheet that has been bent along a bending line, comprising the followingstep: generating at least two flows of compressed air in two matrix diepoints that are totally or partially covered by the plate sheet duringits bending step; measuring the pressure variation between the two flowsof compressed air during said bending step; processing the measuredpressure values and comparing then with predetermined sample valuesthrough calibration.
 13. Process according to claim 12, characterised inthat it comprises: a first flow of compressed air that is directedperpendicular to the plate sheet before its bending and is placed on thematrix die next to the plane sheet-bearing surface next to the matrixdie groove; a second flow directed perpendicular to the slot surface.14. Process according to claim 12, characterised in that it provides formeasuring the pressure of a pair of compressed air flows on both slotfaces.
 15. Process according to claim 12, characterised in that thepressure measure between the two air flows is performed with a pressuretransducer of the differential type, connected to the two pneumaticsupply ducts of a pair of orifices on the matrix die.
 16. Processaccording to claim 12, characterised in that the pressure differencebetween the two air flows can be kept constant at a zero value, by meansof a servo-controlled device comprising two orifices and a shutter,which is able to externally compensate for the covering differences oforifices on the matrix die generated by the plate abutment, andcharacterised in that the angle measure is referred to the positionmeasure of the external compensating device.
 17. Process according toclaim 12, characterised in that, in order to more easily detect thesmall variations with respect to a provided bending angle, theservo-controlled device is kept in a position corresponding to theprovided angle and the pressure difference is measured, provided by thetransducer around the zero value.
 18. Process according to claim 12,characterised in that the common compressed air supply line isperiodically shut off to allow compensating for differential pressuresensor zero errors.
 19. Process according to claim 12, characterised inthat a single pressure measuring system can be used with different pairsof orifices obtained in different matrix dies, by selecting every timethe pairs of orifices, by means of solenoid valves on respectivepneumatic supply ducts.
 20. Process according to claim 12, characterisedin that the pair of orifices for measuring the bending angle is obtainedon the punch.
 21. Device for measuring the bending angle comprising acompressed air system on whose supply line it provides a system pressurereducer; two pairs of orifices arranged on the matrix die face that getin contact with the plate sheet in symmetrical positions with respect tothe vertical axis of the matrix die groove apex, each one of saidorifices being supplied by a respective flow of compressed air,characterised in that the mean of measures of plate bearing angles ontothe matrix die is performed by mutually pneumatically connecting the twoorifices which are external to the matrix die slot and the two orificeswhich are internal to the slot itself.